A reactor of this type generally comprises a core composed of fuel assemblies immersed in a liquid metal such as sodium filling the reactor vessel, this vessel being closed by a very thick slab.
The liquid sodium constitutes the cooling fluid of the core which takes up the heat given off by the fuel assemblies.
There are components which cross the reactor slab and are immersed in the liquid sodium filling the vessel. Thus primary fluid circulation pumps and intermediate exchangers immersed in the liquid sodium make it possible to extract heat from the reactor during the its operation.
In the case where the reactor is shut down by introducing control rods in maximum insertion position into the fuel assemblies, the nuclear reaction stops with the neutrons produced by the core being absorbed by the control rods.
However, the nuclear reactor continues to produce a certain amount of heat, because of the residual activity of the reactor core.
Immediately after shutdown, the power of the reactor does not fall to a zero value but very quickly to a fraction of a percent of the nominal power, and decreased only slowly thereafter.
This residual power for large reactors of 3000 MW thermal power, for example, is nevertheless significant in absolute terms and leads to an increase in the internal temperature which can be dangerous above a certain level.
During the period when it is shut down, the reactor core therefore continues to dissipate a certain quantity of heat due to the nuclear reactions of the materials of which it is composed which have been activated during the operating period of the reactor.
It is necessary to remove the heat thus dissipated by the reactor and taken up by the liquid sodium constituting the primary fluid, in order to avoid an excessive rise in temperature in the interior of the vessel.
This removal of heat is normally provided by the device which exracts the heat from the reactor during operation. This device includes intermediate exchangers which extract the heat from the primary fluid, inside the vessel, by thermal contact of this primary fluid with the secondary fluid, generally liquid sodium, which itself is used to heat feed water and transform it into steam.
This device for removing the heat from the reactor generally comprises several independent assemblies each including a circulating pump, one or more exchangers and a steam generator.
In the case where the reactor is shut down, the function of removing the residual heat from the reactor can be provided by a single assembly of means a type similar type to those described above, since the power to be removed is only a small fraction of the total power of the reactor.
In the case where one or more of these assemblies becomes inoperable, the function can be provided by a single one of these assemblies.
However, in the case where the shutdown of the reactor is a result of a very serious accident to the reactor, it is possible that none of the assemblies for removing the heat from the reactor in normal operation may be available.
On large reactors, therefore a device which is totally independent of the normal cooling circuit of the reactor is provided for the removal of the heat dissipated by the reactor when shut down.
An auxiliary device of this type for the removal of heat includes one or more exchangers of the sodium-sodium type immersed directly in the sodium contained in the reactor vessel and one or more exchangers of the sodium-air type for the cooling of the secondary sodium from the sodium-sodium heat exchanger.
The primary sodium is thus cooled in contact with the secondary sodium in the sodium-sodium heat exchanger, and the secondary sodium is itself cooled inside a sodium-air exchanger before being returned to the sodium-sodium exchanger.
The use of secondary sodium makes it possible to avoid contact between the primary sodium which is contained in the vessel and which has a certain radioactivity, with the cooling air outside the vessel.
In order that a device of this type for removing heat from the reactor when shut down may function in place of the device for normal extraction of the heat from the reactor, after an accident hes put this device for normal extraction out of use, it is necessary that the auxiliary device for removal of the heat has a very reliable operation, has great strength and contains very few active elements.
In fact, in the case of an earthquake having damaged the reactor or in the case of an accident in the core having destroyed the latter in such a way that it is out of operation but still intact, it is necessary to retain a facility for removing the heat due to the residual activity of the reactor core.
Moreover, the sodium-sodium exchangers used for the extraction of the heat dissipated by the reactor when shut down must take up a small amount of room on the reactor slab which carries a large number of components, some of which are very large, such as the primary pumps and the intermediate exchangers which provide the circulation of the primary fluid and the extraction of heat during normal operation of the reactor. The sodium-sodium exchangers of the auxiliary device for removing heat must therefore have a small diameter, for example less than 600 mm.
Heat exchangers are known which comprise an external casing, open at its lower end, immersed in the sodium of the vessel and enclosing a nest of helically wound tubes inside which the secondary cooling sodium circulates. These tubes are connected to two tube plates of which one emerges in the inside of the upper part of the heat exchanger in a zone where the secondary sodium arrives and the other emerges in an extraction zone of the heated secondary sodium.
However, device of this type does not have a sufficient strength to be assured of its operation when subject to large-amplitude external stresses, such as those accompanying an earthquake.
In addition, in a device of this type, the nest of helical tubes occupies a large fraction of the interior volume of the external casing, with the result that the circulation of primary sodium in contact with the tubes of the nest is considerably slowed down. Forced circulation of the sodium inside the casing of the heat exchanger must be provided, and this forced circulation requires active elements such as pumps, in particular of the mechanical type, which can easily be put out of action if an earthquake occurs.